Mr Brehony wrote the letter on February 5 after Mr Calleary put the question to Health Minister James Reilly.

The deputy became concerned after he was contacted by a HSE worker who suspected he had been placed under surveillance.

In a parliamentary question to Dr Reilly, Mr Calleary queried the “amount of money spent by the Health Service Executive in 2010, 2011, 2012 and 2013 on the hiring of private investigators to monitor the movements of HSE staff”.

Response Correspondence to Dara Calleary from HSE

Despite Mr Calleary requesting a statement from the minister on the matter, it was referred to the HSE, who confirmed the practice had taken place.

When questioned on the issue by the Irish Mirror, the health service responded: “It is not the typical practice of the HSE to engage private investigators.

“No further information other than what has already been provided in response to the PQ is available at this time.”

The HSE declined to give a breakdown of the expenses or offer an explanation as to why investigators were used.

It is not clear if the amount spent on private investigators relates to one case or several, but one HSE worker has claimed he was placed under surveillance.

Eugene Rutledge, from Ballyshannon in Co Donegal, worked as a care attendant for HSE West and said he obtained photographs taken of himself by a private investigator after confronting his Employee Relations Officer.

The 51-year-old claims he had his pay suspended in November last year after his employers refused to accept he was suffering from an injury which meant he couldn’t work.

He sustained the injury while on duty for the HSE in April 2011, which left him with a severe urological disorder.